The folds and faults kinematic association in Zagros

The Zagros orogenic belt, one of the most prominent and famous collisional belts in the central part of the Alpine-Himalayan orogenic chain, is located between the southern margin of the Central Iranian microcontinent and the northern margin of the Arabian plate. The structural architecture and folds and faults relationships of a significant segment of the south-central part of the Zagros’ hinterland are investigated in this study through stereoscopy of aerial photographs, interpretations of satellite images, consideration of the major ground topographic variations, and field research. This research found that there must have been at least two major deformation events: (1) a ductile phase, which is older than the Eocene, and (2) a semi-brittle deformation stage, which is younger than the early Miocene and is represented by thrusting, folding, and strike-slip faulting. The presence of numerous fault-related folds and fold-accommodation faults in this area demonstrates the close kinematic relationship between folding and faulting. Based on the topographic changes, a major hidden tear fault and a basement hidden back thrust, which play important roles in the architecture of the area, have been suggested.

Data and method. In order to investigate the structural architecture and particularly the faults and folds relations in the study area, the vertical aerial photographs at 1:50,000 scale, taken from the archives of the Department of Earth Sciences at Shiraz University, and the Landsat 8 satellite (Google Earth) images were considered. Utilizing this data, map-scale structures (e.g., macroscopic faults and folds) have been carefully detected and mapped (Figs. 2, 3). While large-scale structures form the structural framework of the ZHFTB, mesoscopic ones are very valuable because, since the early twentieth century, it has been accepted as Pumpelly's rule that the characteristics of map-scale architecture can be cleared up by surveying more plentiful and accessible associated minor structures 35 . Therefore, intensive field campaigns were conducted in the various parts of the study area, and outcrop-scale structures, including mesoscopic folds, different kinds of mesoscopic faults, and shear zones, were studied. Aerial photography becomes significantly more effective when a stereoscope is used in the field to gain a better understanding of field relationships. Using this advantage, it became clear that the mean elevation

Results
Structural characteristics of the study area. The tectonic structures in the area occurred on a wide range of scales. The characteristics of map-scale structural architecture are formed by the large-scale structures that have been presented on the detailed maps of the study area (Figs. 3,4). The structures that play the most important roles in the tectonic framework of the study area are the various kinds of faults, folds, and shear zones. In this study, the faults were recognized from topographic variations, strike separations, and the bedding attitudes or lithological characteristics observed during the field investigations, as well as from aerial photographs stereoscopy, satellite (Google Earth) image interpretations, and topographic profile considerations. Vertical aerial photographs have been used as a basis for field mapping, plotting dips, strikes, attitudes of faults, and contacts. The characteristics of the map-scale structures can be cleared up by surveying more reachable abundant outcrop-scale structures. In some cases, faults' planes and slickenlines were observed during field campaigns ( Fig. 5A Fig. 5C). There is also a map-scale into-anticline thrust in the Assouk anticline (Fig. 8A). The strike line of this steeply dipping reverse fault is NW-SE, which is parallel to the axial plane of the Assouk anticline and its dip is toward the NE. Even though the strikes of most of the strike-slip faults are N40W, there are some such faults with strikes ranging between N20-70° W in the Faryadoun Mountain (Fig. 3). The strike-slip faults are vertical or semi-vertical and dextral, while the dip of most of the thrusts and reverse faults is 10°-40°.

Topographic step.
A considerable topographic step is also recognizable between the Faryadoun Mountain and its northern area (Dareh-Nar valley and its adjacent areas, i.e., Assouk and Sorkhouy mountains). The average elevations in the north and south of this topographic step on the plotted profiles are 2010 and 2700 m, respectively (Fig. 6). This elevation contrast confirmed that there is a meaningful topographic step between the Faryadoun Mountain in the south and the Dareh-Nar valley in the north. The best explanation for the existence of this topographic step is the activity of a hidden basement SW-dipping thrust. Considering the overall tectonic vergence of the study area, which is to the SW, the NE verging basement fault is a back thrust. This inferred basement fault is named the Dareh-Bagh thrust.    www.nature.com/scientificreports/ and limestones. These mesoscopic symmetric and asymmetric folds have chevron, concentric, and box profiles and mostly consist of thin-bedded limestones and dolostones. The axial planes are generally moderately to steeply dipping (50°-70°), although shallowly dipping (Fig. 9A) and vertical axial planes also occur. Despite the existence of different folds such as upright to recumbent and horizontal to vertical folds, the horizontal inclined folds are the more abundant (Fig. 9B,C). The interlimb angles vary greatly, but most of the outcrop-scale folds in the study area are too tight to open. They are located in the tilted layers, the limbs of the map-scale folds, and even in the brittle-ductile shear zones associated with small-scale faults in these zones (Fig. 9D,E). The trends of the folds' axes, which are located on the tilted layers and the limbs of the map-scale folds, are mostly similar to the map-scale folds (the trends of their low plunge hinge lines are commonly toward NW or SE), which are semi-parallel to the strike of most of the thrust faults of the area (Fig. 10A); while the trends of the folds that are located in the brittle-ductile shear zones depend on the attitude and the sense of movement of the shear zones, the trends of their axes are generally normal to the displacement vectors of the shear zones. In addition, there are some outcrop-scale detachment (decollement) folds (Fig. 9F,G) which developed when displacements along bedding-parallel detachment faults (with no ramp) have been transferred into the folding of the hanging wall blocks. In the southern part of the Dare-Nar valley, there are some tilted (~ 50°) outcrop-scale detachment folds (Fig. 9H) due to thrusting and folding.
Outcrop-scale faults. One of the most observed structures in the field investigations of the Faryadoun and Dareh-Nar areas is the outcrop-scale faults. There are different types of mesoscopic faults in the study area, such as various types of thrust (Figs. 10B, 11A-H, 12A-H), strike-slip (Fig. 13A,B), and normal faults (Fig. 13C,D). The outcrop-scale thrust faults are the most frequent ones, which occurred as the cause and/or result of the mes- www.nature.com/scientificreports/ oscopic folds or without any obvious relationship to them. As a result of the activity of these numerous outcropscale thrust faults, some typical mesoscopic duplex structures formed (Fig. 11F,G). The hinge zones and limbs of the observed symmetric and asymmetric outcrop-scale folds have usually been dissected by mesoscopic thrust faults. The strikes of these thrust faults are mostly parallel to the hinge zones of the mesoscopic folds. These cmdisplacement thrust faults cut only a few layers. As well as the different types of mesoscopic fault-related folds, especially fault bend folds (Fig. 11H), the various kinds of outcrop-scale fold-accommodation faults 7 , such as the back thrusts (Fig. 12A,B), the forelimb thrusts (Fig. 12C), the forelimb space-accommodation thrusts (Fig. 12D), the hinge wedge thrusts (Fig. 12E), the limb wedge thrusts (Fig. 12F), the out-of-syncline thrusts (Fig. 12G) and the into-anticline thrusts (Fig. 12H) are the common structures in the study area. There are a lot of slickensides www.nature.com/scientificreports/ on the contact surfaces of the tilted layers in the limbs of the map-scale folds (Fig. 14). The slickenlines are generally perpendicular to the folds' axes. Similar to the map-scale strike-slip faults, the outcrop-scale ones (Fig. 13A,B) are vertical to semi-vertical, often dextral, and more dominant in the southern parts of the study area (i.e., Faryadoun Mountain), with strikes that are generally similar to the other aforementioned faults (NW-SE). Outcrop-scale normal faults are the other obvious structures in the study area. The prevailing dips of these m-displacement faults are 55°-65°. Their attitudes vary considerably, but the E-striking ones are more frequent.
Layer-parallel ductile shear zones. In the Sorkhouy, Faryadun, and Assouk mountains, there are many layer-parallel narrow ductile shear zones. They are mostly NNW-SSE striking, ENE gently dipping in the Sorkhouy and Assouk mountains (Fig. 15A), and NW-SE striking, SE dipping in the Faryadun mountains (Fig. 15B). These shear zones have been formed in the less competent layers and have facilitated the shearing of the layers. They consist of layers of parallel calcite mylonitic foliation. Deformed chert nodules are plentiful in these shear zones (Fig. 15B). These less competent layers have been eroded more than the other layers (Fig. 15).
Sinistral top-to-the NW deformation. The field observations (Fig. 16) in terms of the fabric elements ( Fig. 16A) and reliable shear sense indicators (Fig. 16B-F) reveal that there is obvious general sinistral top-to-the NW ductile deformation in the different parts of the study area 15,30,[35][36][37][38] .

Discussion
The SSZ is the inner part of the Zagros collisional zone which is deformed severely 39 . It has been developed as a consequence of the subduction of the Neotethyan oceanic plate under the Central Iranian microplate in the Jurassic to Cenozoic and the subsequent collision between this and the Arabian plate in the middle of the Cenozoic 20,39 . The SSZ has been divided into the southwestern metamorphic zone (i.e., SSMB) and the northeastern metasedimentary belt (i.e., ZHFTB) 14,30 . Almost all of the study area, which is the south-central part of the ZHFTB, is affected by low-grade metamorphism, except the units younger than the Early Eocene. According to the age dating results of the Sanandaj-Sirjan Zone 40 , the metamorphism of the area is considered to be older than the Eocene. There is at least one sinistral top-to-the NW shearing ( Fig. 16) 30,35,36,38 which is older than Neogene 40 . This sinistral top-to-the NW deformation is not restricted to this area 15,41 . The fabric of Heneshk (Kowlikosh) shear zones (Fig. 12 in 42 ) and the Neyriz area (Fig. 3 in 43 ), also, indicate the occurrence of sinistral deformation in the different parts of SSMB.
In the ZHFTB, the two architectural structures are faults and folds. In addition to the exposed macroscopic faults and folds, which have been presented on the maps (Figs. 2, 4), two hidden faults have been introduced in this study: the Dare-Nar tear fault along the linear Dare-Nar valley (Figs. 2, 3), and the Dare-Bagh back thrust, which resulted in a prominent topographic step in the north of the Faryadun mountain. The effects of hidden basement faults on the surface topography have not been presented before in this area but have been reported in many other areas, such as the Darang and Surmeh anticlines in the Zagros Foreland Folded Belt 44 . The cause of the existence of the Dare-Nar valley seems to be a NE-SW striking tear fault (i.e., the Dare-Nar tear fault), similar to the Talaee tear fault, which was introduced by Sarkarinejad and Ghanbarian 14 . The vertical offset, which is caused by the Dare-Nar tear fault, is much less than the offset of the Talaee tear fault. The relationships of the faults and folds in the outcrop-scale fault-bend folds and fold accommodation faults are more obvious than their association in the map-scale structures. The map-scale thrust faults on the limbs of the map-scale folds, however, suggest that these folds could develop as thrust-related folds 45,46 . Thus, the fault-related folds were observed at different scales and in the various types of fault-bend folds and asymmetric detachment folds. Several incompetent layers, such as the Upper Devonian metaterrigenous unit (D sh ), the Lower Permian sandstone and shale (P s ), and the Lower Triassic thinly-bedded marl and limestone (Tr l ), acted as the detachment surfaces and facilitated occurrences of the thrust systems, detachment folds, and layer parallel shear zones. There is no evidence of a map-scale roof thrust, so the thrust systems are mostly imbricate fans. This is in contrast with the idea of Sarkarinejad and Ghanbarian 14 , which suggested that the structural architecture of the area is characterized by several duplex structures. Several successive map-scale foreland dipping horses, however, occurred in www.nature.com/scientificreports/ the center of the study area and north of the Faryadun mountain (Fig. 3). Therefore, the thrust faults play the most determining roles in the structural architecture of the study area, and many other structures, such as folds, have occurred as a result of their development. Sarkarinejad and Ghanbarian 14 emphasized the determining role of the map-scale forethrusts, too. The map-scale back thrusts of the study area ( Fig. 7C-F), which are very significant structures in the area (Fig. 3), can occur due to the flat-ramp geometry of a main underground fore thrust, the occurrence of pop-up structures, and the steepening of the underlying foreland-dipping duplexes (due to ongoing shortening of the area), which bend the flat thrusts to the SW-dipping thrust. These back thrusts can also be the continuation of the Dareh-Bagh basement back thrust, which caused the main topographic contrast in the vicinity of the main exposed back thrusts of the study area (i.e., the north of the Faryadun mountain; www.nature.com/scientificreports/ Fig. 3). Within the folds of the study area, there are different kinds of faults that have developed due to folding processes. Forelimb thrusts, forelimb space-accommodation thrusts, back thrusts, out-of-syncline and intoanticline thrusts, wedge thrusts including hinge wedges and limb wedges thrusts are the diverse modes of the fold accommodation faults of the study area. Nevertheless, the geometric and kinematic relationship between thrust faults and folds are scale-invariant, as discussed by Sarkarinejad and Ghanbarian 14 . The existence of many slickensides between tilted layers in the macroscopic folds' limbs suggests that there are layer parallel slips that developed during flexural slip folding. The Upper Oligocene-Lower Miocene reefal limestones are involved in younger than the earliest Miocene thrusting. The tilted solution pits on the surfaces of the Permian limestone layers in the Faryadoun Mountain (Fig. 8F) indicate that the fault-related folding in the area is very young, too. Some dextral strike-slip faults have cut the northern parts of Faryadoun Mountain (Fig. 3), a phenomenon that is well-documented by Nadimi and   www.nature.com/scientificreports/ Konon 47 in the Esfahan region. There is no apparent kinematic relationship between the strike-slip faults and folds. Therefore, there must be at least two major events, (1) a ductile one older than the Eocene, which is accompanied by metamorphism and sinistral top-to-the NW deformation, and (2) a semi-brittle deformation younger than the earliest Miocene. The latter is represented by folding, thrusting, and strike-slip faulting. The folding and faulting in this event are interrelated, as shown by the existence of various fault-related folds and fold-related faults in the area. This kinematic association suggests that this second deformation event is not completely brittle.

Conclusion
The structural analysis of this part of the ZHFTB was based on aerial photographs stereoscopy, satellite (Google Earth) images interpretations, consideration of the major topographic changes, and field investigations. The results of this research revealed that there must be at least two main deformation phases, (1) a ductile event, which is older than Eocene and accompanied by metamorphism and sinistral top-to-the NW deformation, and (2) a semi-brittle deformation phase that is younger than the earliest Miocene and is represented by thrusting, folding, and strike-slip faulting. The abundant fault-related folds and fold-accommodation faults of the study area reveal the close connection between folding and faulting in this region. A basement hidden back thrust and a major hidden tear fault has been introduced in this study based on the surface topographic changes. The basement hidden back thrust may be the cause of the map-scale back thrusts and other structures in the north of Faryadun Mountain.